1. Field of the Invention
The present invention relates to a device and method for reading information stored in a semiconductor device, and more particularly, to a data sensing device and method to be used with a multibit memory cell for reading data programmed in multilevel and stored in the multibit memory cell.
2. Discussion of the Related Art
Semiconductor memories generally include volatile memories and non-volatile memories. For volatile memories, information can be recorded and later erased. For non-volatile memories, once information is recorded, it can be conserved permanently. Volatile memories generally include RAM to/from which data is recordable and readable. Non-volatile memories include ROM, erasable programmable ROM (EPROM), and electrically erasable programmable ROM (EEPROM). ROM is a memory which is no longer programmable once information is recorded. EPROM and EEPROM are memories from/to which information is erasable and recordable. EPROM and EEPROM are identical in their programming operations, but different in their information erasing operations. Information recorded in EPROM is erased by an ultra-violet beam and information stored in EEPROM is erased electrically.
Of all memories, dynamic random access memory (DRAM) is the most widely used as a mass storage media because it provides large sized memories which are required by the information industry. However, DRAM requires a large size storage capacitor. In addition, it has the disadvantage that refreshing operations of the capacitor are required in certain intervals.
As a result, there have been many studies on using EEPROM instead of DRAM, since EEPROM does not require any refreshing operations. However, since EEPROM generally stores data of either "1" or "0" in a memory cell, its device packing density is related to the number of the memory cells in one to one fashion. Therefore, the drawback in using EEPROM as a data storage media is the high cost per bit of memory.
To solve this problem, active studies on multibit-per-cell devices are currently underway. A multibit memory cell stores data of over two bits in one memory cell, thus enhancing the data density on a single chip area without increasing the size of the memory cell. For the multibit memory cell, more than two threshold voltage levels should be programmed on a respective cell. For instance, in order to store two bits of data for every cell, the respective cells must be programmed in 2.sup.2 (i.e., 4) threshold levels. Here, the four threshold levels correspond to logic states 00, 01, 10 and 11, respectively. By precisely adjusting the respective threshold levels, more levels can be programmed, which in turn increases the number of bits for every cell. As a result, the data thus programmed in multilevels can be read in quickly.
A conventional data sensing device for sensing data programmed in multilevels will now be explained with reference to the attached drawings. FIG. 1 illustrates a system including a conventional sensing device for sensing a multibit memory cell, and FIG. 2 is a graph showing the operation of the conventional sensing device. Generally, when voltages which are large enough to cause reading are applied to a control gate, a current flows between a drain and a source. The current is then compared to a reference current and the comparison result determines the data reading operation. Referring to FIG. 1, a conventional system includes a sensing amplifier (SA) connected to a drain region D in a unit cell of an EEPROM, and the unit cell has a floating gate FG, a control gate CG, a source region S and the drain region D. The sensing amplifier SA has a plurality of reference currents therein.
A method of sensing data in a multibit memory cell having the aforementioned conventional system will now be explained.
It is assumed that the memory cell has been programmed in multilevel threshold voltages. That is, as shown in FIG. 2, when recording a two-bit data, it is assumed that the two-bit data has being programmed in the floating gate FG as one of four threshold voltages V.sub.T0, V.sub.T1, V.sub.T3, VT.sub.4. When a constant voltage is applied to the source region S, a predetermined voltage V.sub.READ is selectively applied to the control gate CG of the memory cell. Then, according to the programmed state of the floating gate FG, a drain current I.sub.D (see FIG. 1) corresponding to the programmed state flows through the sensing amplifier SA. The sensing amplifier SA compares the drain current I.sub.D from the memory cell to the multilevel reference currents within the sensing amplifier SA, thereby reading the data. That is, referring to FIG. 2, if the floating gate FG of a memory cell in an EEPROM has been programmed at the threshold voltage V.sub.T0, a drain current I.sub.R0 corresponding to the threshold voltage V.sub.T0 will flow through the sensing amplifier SA. Similarly, if the floating gate FG has been programmed at the threshold voltage V.sub.T1, a drain current I.sub.R1 corresponding to the threshold voltage V.sub.T1 will flow through the sensing amplifier SA. If the floating gate FG has been programmed at the threshold voltage V.sub.T2, a drain current I.sub.R2 corresponding to the threshold voltage V.sub.T2 will flow through the sensing amplifier SA. Therefore, upon receiving the drain current from the drain of the memory cell, the sensing amplifier SA senses the data by comparing the drain current with the multilevel reference currents within the sensing amplifier SA.
Because a predetermined voltage Vc of a readable condition is applied to the control gate of a memory cell from which a data is to be read, and a multilevel comparison of the drain current from the memory cell is performed in the sensing amplifier in reading the data, the conventional data sensing device and method for sensing a data in a multibit memory cell have the following problems.
First, to perform the multilevel comparison of the drain current in the sensing amplifier when reading the data, the sensing amplifier must have the multilevel reference currents. This increases the size of the sensing amplifier, particularly in a page mode READ, which requires more bits (for example, 512 bits, 128 bits). Consequently, the chip size is increased. Second, the required multilevel reference currents in the sensing amplifier increase the overall power consumption.